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Black hole pair
A combination of X-ray and optical imagery shows the black hole pair known as J0045+41 glowing amid the much closer stars of the Andromeda Galaxy. (X-ray: NASA / CXC / UW / Dorn-Wallenstein et al. Optical: NASA / ESA / J. Dalcanton et al. and R. Gendler)

It turns out that even galaxies can be photobombed.

Imagery from NASA’s Chandra X-ray Observatory and ground-based telescopes reveal what researchers say could be the closest-orbiting pair of supermassive black holes ever seen.

Scientists have long known about the black hole pair, which is designated with the unwieldy label LGGS J004527.30+413254.3 (or J0045+41 for short). But they assumed it was an object within the Andromeda Galaxy, also known as M31. They thought it might have been a pair of eclipsing binary stars.

Then a team of astronomers from the University of Washington took a closer look at J0045+41 during their search for red supergiant X-ray binary stars.

“We were looking for a special type of star in M31, and thought we had found one,” UW’s Trevor Dorn-Wallenstein, the lead author of a research paper on J0045+41, said today in a news release. “We were surprised and excited to find something far stranger.”

Chandra’s X-ray data about the object showed that the stellar system had to have a black hole or neutron star. Additional spectral data from the Gemini North Telescope in Hawaii provided evidence that J0045+41 had to host at least one supermassive black hole, and perhaps two.

The Gemini North data also provided enough clues to estimate the object’s distance. Rather than being embedded within the Andromeda Galaxy, a mere 2.5 million light-years from Earth, the object was 1,000 times farther away, at a distance of 2.6 billion light-years.

Further optical data from Caltech’s Palomar Transient Factory showed that there were several periodic variations in the light from J0045+41, including 82-day and 328-day cycles. The patterns in the variations fit theoretical models for the dynamics of two mutually orbiting black holes.

“This is the first time such strong evidence has been found for a pair of orbiting giant black holes,” said UW’s Emily Levesque, a co-author of the study.

The evidence suggests that the two stars are separated by somewhere between 200 and 550 astronomical units, or AU, where 1 AU represents the distance between Earth and our sun. That’s roughly 10 times as far away as Pluto is from the sun, but startlingly close when it comes to stellar distances. The distance between our own sun and the next closest star, for instance, is hundreds of times wider.

The two supermassive black holes are almost certainly being drawn together by mutual gravitational attraction.

“We’re unable to pinpoint exactly how much mass each of these black holes contains,” said UW’s John Ruan, another study co-author. “Depending on that, we think this pair will collide and merge into one black hole in as little as 350 years, or as much as 360,000 years.”

The trio’s research paper, titled “A Mote in Andromeda’s Disk: A Misidentified Periodic AGN Behind M31,” was published in the Astrophysical Journal.

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